HR: 0800h
AN: OS51C-0572    [Abstracts]
TI: Nutrient Subsidies to Hanalei Bay, Kauai, HI From Submarine Groundwater Discharge
AU: * Knee, K
EM: klknee@stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Santoro, A
EM: asantoro@stanford.edu
AF: Environmental Engineering and Science, Terman Engineering Center, B-9 Stanford University, Stanford, CA 94305 United States
AU: Street, J
EM: jstreet@stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AU: Boehm, A
EM: aboehm@stanford.edu
AF: Environmental Engineering and Science, Terman Engineering Center, B-9 Stanford University, Stanford, CA 94305 United States
AU: Berg, C
EM: cberg@pixi.net
AF: Hanalei Watershed Hui, PO Box 1285, Hanalei, HI 96714 United States
AU: Paytan, A
EM: apaytan@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 United States
AB: Submarine groundwater discharge (SGD) has been shown to be a potentially important source of freshwater, nutrients, and pollutants to many coastal areas, including some locations in Hawaii. This study investigated the importance of SGD in Hanalei Bay, a northward-opening, half-moon shaped bay approximately 2 km in diameter, located on the northern shore of the island of Kauai. High fecal indicator bacteria (FIB) counts at Hanalei beaches sparked concern that nutrients and/or bacteria might be leaching into groundwater from septic systems and cesspools and making their way to the coastal ocean. Sampling was conducted in March and June 2005 at 3 local beaches, 3 streams, the Hanalei River, several groundwater pits, and the open bay. Radium was used as a groundwater tracer. Salinity, water temperature, nitrate, nitrite, ammonium, phosphate, silicate, total coliform, Enterococcus, and E. coli were measured. The ratio of 223Ra (half life = 11.4 days) to 224Ra (half life = 3.66 days) was relatively constant across samples from groundwater and the bay, indicating that the residence time of water in the bay was less than one day during the study period. Groundwater had lower concentrations of all FIB than the Hanalei River, the streams, or the bay, indicating that SGD was not contributing bacteria directly to the coastal zone over the course of this study. However, E. coli was detected at relative high levels in groundwater seaward of a cesspool, suggesting that during periods of high discharge, SGD could transport fecal bacteria to the bay. Concentrations of all nutrients were higher in groundwater than in Hanalei Bay, and nitrate concentrations were higher in groundwater than in the Hanalei River or the streams. Additionally, nitrate concentrations in groundwater and in Hanalei Bay were coupled with radium activities, suggesting a common source, most likely SGD. Mass balance calculations, based on residence times of 2 and 6 hours for the surf zone, indicate that the SGD flux into Hanalei Bay may be as high as 1200 L/s, or 20 percent of the Hanalei River's average March flow rate, and that SGD may contribute more nitrate to Hanalei Bay than does the Hanalei River. These nutrient subsidies could impact nearshore ecosystems by promoting the growth of bacteria and/or algae. Further research is needed to investigate the spatial and temporal variability of SGD in Hanalei Bay, the residence time of water in the Bay, nutrient limitation of bacteria and algae, and the relative importance of various nutrient sources such as cesspools, taro fields, natural soil nitrogen fixation, and urban runoff.
DE: 0478 Pollution: urban, regional and global (0345, 4251)
DE: 1831 Groundwater quality
DE: 4217 Coastal processes
DE: 4220 Coral reef systems (4916)
DE: 4845 Nutrients and nutrient cycling (0470, 1050)
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005